Stripping excess coating liquid from moving strip materials

ABSTRACT

METHOD AND APPARATUS FOR STRIPPING EXCESS COATING LIQUID FROM AN UPWARDLY MOVING STRIP WITH GS STREAMS, AND REDUCING THE RIPPLE DEVELOPING EFFECT OF THE UPWARD COMPONENT OF GAS STREAMS IMPINGING ON THE COATED STRIP BY DISPERSING OR SUPPRESSING PART OF THE UPWARD COMPONENT WHICH FORMERLY REMAINED ADJACENT THE SURFACE OF THE STRIP BY PLACING A STREAM-FLOW MODIFYING ELEMENT IN THE PATH OF THE UPWARD COMPONENT.

P. REID ETAL 3,681,119

STRIPPING EXCESS COATING LIQUID FROM MOVING STRIP'MATERIALS Aug. 1, 1972 2 Sheets-Shed 1 Filed May 26, 1970 P. REID E'TAL STRIPPING EXCESS COATING LIQUID FROM MOVING STRIP MATERIALS Filed May 26. 1970 2 Sheets-Sheet 2 United States Patent 3,681,119 STRIPPING EXCESS COATING LIQUID FROM MOVING STRIP MATERIALS Paul Reid and Brian James, New Lambton, New South Wales, Australia, assignors to John Lysaght (Australia) Limited, Sydney, New South Wales, Australia Filed May 26, 1970, Ser. No. 40,509 Int. Cl. B05c 11/06 US. Cl. 117-102 L 5 Claims ABSTRACT OF THE DISCLOSURE Method and apparatus for stripping excess coating liquid from an upwardly moving strip with gas streams, and reducing the ripple developing effect of the upward component of gas streams impinging on the coated strip by dispersing or suppressing part of the upward component which formerly remained adjacent the surface of the strip by placing a stream-flow modifying element in the path of the upward component.

It is commonplace to strip excess coating liquid from continuous strip material, as the material rises out of a dipping bath or other coating station, by directing gas streams or jets onto the coated surfaces in a direction substantially normal thereto. The gas is usually air, superheated steam or other suitable media. The coating liquid may be paint, glue, molten metal or other common coating liquid and the strip (to which the particular coating must of course be appropriate) may be paper, sheet metal or other strip material pliant enough for reeling and amenable to production in long runs; for example, lengths running to hundreds of feet.

Although the present invention is applicable to jet stripping in the general sense indicated above, it is primarily concerned with the stripping of molten coating metal from strip steel as it emerges from a molten metal bath; and will be described hereinafter mainly in terms of such a use; namely the zinc coating of strip steel as it emerges from a galvanizing bath.

In the galvanizing of strip sheet steel, one of the generally preferred methods of the prior art, to remove excess zinc from the steel, is to arrange for the coated strip as it leaves the molten zinc bath to move vertically through a zone containing jet stripping means and, once the coating has solidified the galvanized strip is reeled.

The preferred jet stripping means comprise gas nozzles disposed, substantially horizontally, above the surface of the coating bath; these nozzles supply a stream of gas across the full width of the strip, and on both surfaces thereof, so that the desired thickness of zinc will be left on the steel. Thickness of coating is controllable by varying the distance between the nozzle slots and the steel strip, or by varying the intensity of the gas flow out of the nozzles.

The jet stripping procedure referred to above is effective and has largely superseded other ways of stripping, but it is open to objection in several respects. The most serious of these disabilities is what is commonly known as rippling; that is, formation of loop-like or wavy formations of the molten metal which solidify before levelling out thus giving unevenness of coating to a degree which is noticeable in the finished product, especially where the surface is required to be painted or similarly smoothly finished.

Another disability, quite apart from rippling, is that by the existing methods coating unevenness occurs; this is not noticeable in the same way as rippling, but is nevertheless objectionable principally because in the example quoted, the zinc coating is used for corrosion protection and uneven coatings give variable corrosion resistance. A further objectionable feature of the prior art practice is that known as edge build-up. This is manifested as excessive thickness of coating metal in the vicinity of the longitudinal edges of the steel strip.

The main object of the present invention is to eliminate or reduce the incidence of rippling. However, it is also an object of the invention, which applies to preferred embodiments thereof, to eliminate or ameliorate unevenness of coating (apart from rippling), and eliminate or reduce edge build-up.

For efliciency of stripping it is necessary for the stripping gas jets to be directed substantially normal to the surface of the coated strip and this results in the gas streams, where they impinge upon the strip (hereinafter referred to as the stripping station) dividing into two components respectively directed generally upwardly and downwardly. Our experiments have shown that the impact of the gas is responsible for effectively thinning the coating to the desired thickness whereby substantially all the excess molten metal is swept downwardly and the downward component of the gas stream once the excess coating metal has been so displaced is then able to sweep most of the displaced metal downwardly, somewhat in the form of waves and this downward displacement is assisted by gravity. The now coated strip, carrying the desired thickness of coating metal, in moving from the stripping station is subject to the action of the upward component of the gas stream and our experiments have shown that it is this action of the upward gas jet stream which is responsible for the development and sustaining of ripples. Thus the process of this invention is primarily directed to reducing what may be called the ripple sustaining development action of the upward component of the gas stream by dispersing or suppressing a substantial proportion of the upward component that would otherwise remain adjacent the surface of the strip; the upward component thus becomes less effective in the development and sustaining of ripples or other similarly caused coating unevenness.

While it is established in the prior art that said gas streams should be directed substantially normal to the strip movement, gas streams at other angles to the coated strip have been employed. but these also break up into upward and downward components as hereinbefore described and the adverse effects of the upward component still apply.

General unevenness of coating (apart from rippling) appears to be due to two factors, both of which impair constancy of spacing between the gas jet nozzles and the strip surface in the stripping station. The first of these factors causing unevenness is due to lack of flatness in the strip itself, it being a very difiicult matter to produce strip in commercial quantities which is perfectly flat. The second factor resides in what may be called the flap movability of the through-going strip relative to the jet nozzles on either side of it. With regard to the second of these factors it will be appreciated that once the strip emerges from the galvanizing bath it has not hitherto been practical to give it lateral support until such time as the coating has solidified, thus there is almost invariably some element of flap movement laterally of the stripping station so that portions of the through-going strip advances towards one nozzle while receding from the other, so to vary the uniformity of excess metal removal.

(It has been proposed to provide steadying rollers, pads and the like designed to prevent flap movement of the strip but as they have to be applied only where the coating has solidified or below the molten metal bath surface, they do not provide complete lateral stability with in the stripping station and those disposed above the stripping station have a further disability in that because of their physical contact with the strip, foreign matter picked up by the rollers or pads tends to mark the surface of the coating.

So far as edge build-up is concerned, we have not fully ascertained the reasons as to why this occurs beyond it having been observed that where the two gas streams meet without the strip being between them; that is, just beyond the side edges of the strip, the flow direction and characteristics of the jet streams are affected and considerable turbulence arises in that vicinity, it would appear that this impairs the stripping action a short distance from the edges of the strip; but, whatever the cause of edge build-up, it is found that by practicing the present invention the incidence of edge build-up is reduced.

In its broadest aspect the invention is definable as a method of stripping excess coating liquid from an upwardly moving strip comprising the steps of directing laterally continuous gas streams on to opposite sides of the coated strip in a stripping station so that each gas stream divides into an upward component and a downward component, and reducing the liquid coating disturbance effectiveness of said upward component leaving said station by at least 25% before it arrives at a point above said station at which said coating solidifies.

Examples of apparatus for performing a method according hereto are illustrated in the drawings herewith. Each of the six figures of the drawings is a schematic showing of a stripping station in cross-sectional end elevation. All six figures respectively show different forms of apparatus and FIG. 2 in showing one of those forms also shows a minor modification of it.

In all forms of the invention the main object thereof is achieved by depriving the mentioned upward component of the gas stream with a substantial fraction (at least 25%) of what may be called its liquid coating disturbance effectiveness while it is proceeding from the stripping station to that point above the station at which the strip has cooled sufficiently for the coating remaining on it to solidify. The term liquid coating disturbance effectiveness is used herein in the sense that a gas stream impinging on a liquid coated surface (either in desirable stripping or that which precedes rippling) causes disturbance of the coating liquid; that is movement of the liquid relative to the strip surface which carries it.

In each of the illustrated embodiments, the liquid disturbance effectiveness of the upward gas stream component is reduced by placement in the path of the component of a stream-flow modifying element which may take any one of several forms.

In the arrangement of FIG. 1 the stream-flow modifying element consists of a pair of vanes 7 which extend horizontally and laterally of the rising strip 8. These vanes, of hockey-stick cross-sectional shape, have peaks 9 which are preferably placed as close together as is compatible with a sufiiciency of mechanical clearance for the strip passing therebetween. t

The lower edges 10 are placed a short distance above the stripping station 11 (due to gas issue nozzle 12) and from their peaks 9 the vanes diverge upwardly as shown.

The distance of the lower edge of each vane from the stripping station is governed by the distance above that station at which the coating is still liquid. In the case of the example given herein, namely hot dip strip galvanizing, the distance is preferably not more than 8", but for other coatings the distance may differ. The two vanes are the same in size, action and position hence only one need be described. The vane first splits off a fraction of the upward component 13 of the jet stream so that some of that stream is initially deflected away from the through-going strip 8 and the remainder of the upward component after passing between the lower edge of the vane and the strip is dispersed in the upwardly diverging space between the vane and the through-going strip.

Vanes 7 may be fixed in position once the optimum positioning thereof has been determined.

In practice it has been found that when the gas stream has passed peak 9 a sufiicient proportion of it tends to cling to the vane surface so that a part of it (represented by span D) ceases to be, or is less effective in point of liquid coating disturbance while the remainder of the stream (span R) is insufficiently effective in this respect to cause ripple formation.

If desired the vanes may be hinge mounted and furnished with adjustment means whereby they may be tuned into a position in which ripple formation is no longer apparent. Alternatively, the two vanes are so placed that their upper ends are as close as compatible with a sufiiciency of mechanical clearance for the strip and from their upper edges diverge downwards and reach down to, or almost down to, the jet nozzles 12. They then cause a sufficient part of the upward component of the jets to be turned downwardly for recombination with the issuing gas jet being entrained thereby sufficiently for departure from the stripping station as a part of the downward gas component.

If so desired either the upwardly or downwardly divergent vanes may be mounted, for example on fulcrum pins or other pivot supports, and loaded, for example by springs, in such a manner that reaction against the upward component of the gas stream causes them to main-- tain a constant distance from the up-going strip. Such regulation of distance is not limited to pivots but may be achieved by lateral movement and the like according to methods which will be apparent to those skilled in the art.

Referring to that part of FIG. 2 on the left of the strip 8A, the vane 7A is hinge mounted, at 14, on a grille bracket 15 fixed to the gas issue nozzle 12A. The vane is connected by link 16 to a cap 17 freely rotatable. on and relative to a screwed stem 18 which runs in a stationarily mounted nut block 19 so that, by turning stem 18 the position of the vane may be adjusted to tune out ripple formation by visual inspection.

The right hand portion of FIG. 2 virtually repeats the left hand portion except for its showing of a tension spring 20 instead of a fixed length link (such as 16). The spring connection permits the vane to adjust itself floatingly in accordance with the pressure prevailing between it and the strip 8A.

In FIG. 3 the stream-flow modifying element is in the form of a vane 21 which acts more as a direct throttle on the upward gas component than as providing a divergent surface which will induce the upward component to spread in the manner indicated by spans D and R in FIG. 1. Vane 21 is fixed on the gas nozzle 12B so to form with it a more-or-less closed space in which a sufficient fraction of the initially upward moving gas component is directed back to the downward component to become a part thereof.

Referring to FIG. 4, the stream-flow modifying element is in the form of a curved vane 22 so that the minimum spacing between it and the strip 8B occurs just above the stripping station 11B. Vane 22 is mounted on the nozzle 12C and forms what in effect is a venturi throat at 23. In this form of the invention some of the upward gas stream is turned downwardly for recombination with the issuing jet to become a part of the downward component. The remainder of the upward component proceeds through the venturi throat and its disturbance effectiveness is lowered as explained above in terms of spans D and R in FIG. 1.

The effect of the venturi throat 23 is to increase the velocity of the gas flowing through it with corresponding loss of pressure therein, and although the reason for it has not yet been determined, experiment has confirmed that the presence of the venturi throat contributes to reduction of the disturbance effectiveness of the upward component.

Where vanes such as 22 are employed they may be hingedly mounted on nozzle 12C and this provides additional significant advantages. Firstly, the flow of the upward component of the gas stream over the lower divergent and upper divergent portions of vane 22 results in generation of forces which cause the vane to be selflocating with respect to distance from the strip determined at the minimum position and to be held in this location relative to the strip without requiring the application of external force to the vane. In other words, the vane is able to follow flap excursions of the strip in either direction from the mean path of travel, maintaining its distance from the strip. Some control over the distance of the vane from the strip can be achieved by varying the lengths and angles of the lower and upper divergent parts of the vane. If it is desired to further adjust the distance of the vane from the strip, an external force may be applied to achieve this.

Secondly, the forces causing the vane to be self-locating are also transmitted to the strip without mechanical contact and can be utilized to locate the strip in the stripping station which is immediately beneath, thereby reducing the flap movability previously described. The advantages of reduction of the strip flap movability are, of course, a reduction in the coating unevenness described earlier and hence are a particular advantage of this form of the invention. Additional forces, e.g. by spring loading, may be applied to the vanes to increase the strip locating character of the vane if so desired. As stated earlier, such forces are transmitted to the strip without mechanical contact.

The arrangement shown in FIG. is similar to that shown in FIG. 4 if vane 22 is regarded as fixedly mounted on the nozzle. In FIG. 5 the stream-flow modifying element consists of an arcuate head 24 which overhangs the lower lip 25 of the nozzle 12D. This form of the invention relies for its effect largely upon the venturi efiect arising in throat 23A.

In the embodiment shown in FIG. 6 the stream-flow modifying elements are in the form of cylindrical rollers or pipes 26 which extend transversely of the strip and beyond its side edges. These rollers are preferably freely rollable by their ends on sloping runway tracks 27 at either side of the strip. These tracks leave the rollers virtually free to roll towards or away from the strip. The rollers may be loaded entirely by their own weights so that during non-working periods they will rest on the tracks, but when the jet streams are started the upward components of the streams are largely obstructed by the roller and hence a proportion thereof is redirected downwardly for amalgamation with the down-going components of the streams. At the same time the obstructed fraction of each of the upward components furnishes enough upward pressure to support the rollers floatingly, as shown in FIG. 6, at the same time providing sufficient space between the rollers and the through-going strip for upward escape of a fraction of each of the upward components which are slowed down and dispersed in a similar way to that due to the 'vanes hereinbefore described. Although the rollers are unrestrained as to movability to the extent indicated above, they are, of course, restrained against endwise axial movement and if desired they may be further restrained by being swing mounted on substantially horizontal radius rods which will permit the rollers to rise and fall fioatingly and at the same time provide a simple means whereby adjustable resilient loading devices may be applied either to reduce or increase the gravity loading of the rollers so to enable the spacing between the rollers and the strip to be tuned as may be required.

We claim:

1. A method of stripping excess coating liquid from an upwardly moving liquid coated strip, comprising the steps of laterally directing gas streams onto opposite sides of the coated strip in a stripping station so that each gas stream divides into an upward component and a downward component, and before said upward component arrives at a point above said station at which said coating solidifies, redirecting at least 25% of said upward component so that the upward component is substantially deprived of liquid coating distunbance effectiveness, wherein said fraction is redirected partly by being split off from said upward component by a stream-flow modifying element placed in the path of said upward component, and partly by clinging to the surface of said element.

2. A method according to claim 1 wherein said fraction is re-directed partly by being turned downwardly for addition to said downward component by the stream-flow modifying element in the path of said upward component, and partly by clinging to the surface of said element.

3. A method according to claim 1 wherein said fraction is redirected partly by proceeding through a venturi throat comprising said strip and the stream-flow modifying element in the path of said upward component.

4. A method according to claim 1, wherein the upwardly moving liquid coated strip is substantially laterally unsupported during its passage through the stripping station.

5. A method of stripping excess coating liquid from an upwardly moving liquid coated strip, comprising the steps of laterally directing gas streams onto opposite sides of the coated strip in a stripping station so that each gas stream divides into an upward component and a downward component, and before said upward component arri'ves at a point above said station at which said coating solidifies, redirecting at least 25 of said upward component so that the upward component is substantially deprived of liquid coating disturbance effectiveness, wherein said fraction is at least in part redirected from said upward component by placing a stream-flow modifying member adjacent said strip in the path of said upward component, which member causes said fraction to follow an aerodynamic path which diverges from said path of said upward component as said component moves downstream past said modifying member.

References Cited UNITED STATES PATENTS 3,499,418 3/1970 Mayhew 117102 MX 3,336,899 8/1967 Mahoney 11863 3,239,863 3/1966 Gardner 15-345 X 2,135,406 11/1938 MacDonald 117102 LX 2,176,093 10/1939 Merrill 117-102 LX 2,536,186 1/1951 Keller 117102 MX 2,992,941 7/1961 Whitley et al. 117--114CX 2,062,795 12/1936 Pike 117114 R 3,459,587 8/1969 Hunter et al 117102 M 3,607,366 9/1971 Kurokawa 117-402 M ALFRED L. LEAVITT, Primary Examiner I. R. BATTEN, JR., Assistant Examiner US. Cl. X.R.

117-102 M, 114 R, 114A; 118-63 

